An optical waveguide mixes and directs light emitted by one or more light sources, such as one or more light emitting diodes (LED elements). A typical optical waveguide includes three main components: one or more coupling surfaces or elements, one or more distribution elements, and one or more extraction elements. The coupling component(s) direct light into the distribution element(s), and condition the light to interact with the subsequent components. The one or more distribution elements control how light flows through the waveguide and is dependent on the waveguide geometry and material. The extraction element(s) determine how light is removed by controlling where and in what direction the light exits the waveguide.
When designing a coupling element, the primary considerations are: maximizing the efficiency of light transfer from the source into the waveguide; controlling the location of light injected into the waveguide; and controlling the angular distribution of the light in the waveguide. The coupling element of a waveguide may be comprised of one or more of a number of optical elements, including a ‘primary’ source optic (such as the lens on an LED component package), one or more intermediate optical elements (such as a lens or array of lenses) interposed between the source and the waveguide coupling surface or surfaces, one or more reflective or scattering surfaces surrounding the sources, and specific optical geometries formed in the waveguide coupling surfaces themselves. Proper design of the elements that comprise the coupling element can provide control over the spatial and angular spread of light within the waveguide (and thus how the light interacts with the extraction elements), maximize the coupling efficiency of light into the waveguide, and improve the mixing of light from various sources within the waveguide (which is particularly important when the color from the sources varies—either by design or due to normal bin-to-bin variation in lighting components). The elements of the waveguide coupling system can use reflection, refraction, total internal reflection, and surface or volume scattering to control the distribution of light injected into the waveguide.
To increase the coupling of light from a light source into a waveguide, it is desirable to maximize the number of light rays emitted by the source(s) that impinge directly upon the waveguide coupling surface. Light rays that are not directly incident on the waveguide from the source must undergo one or more reflections or scattering events prior to reaching the waveguide coupling surface. Each such ray is thus subject to absorption at each reflection or scattering event, leading to light loss and inefficiencies. Further, each ray that is incident on the coupling surface has a portion that is reflected (Fresnel reflection) and a portion that is transmitted into the waveguide. The percentage that is reflected is smallest when the ray strikes the coupling surface at an angle of incidence relative to the surface normal close to zero (i.e., approximately normal to the surface). The percentage that is reflected is largest when the ray is incident at a large angle relative to the surface normal of the coupling surface (i.e., approximately parallel to the surface). To increase efficiency, the coupling of the light into the waveguide body minimizes the absorbing of light at reflection or scattering events as well as the Fresnel reflection at the coupling surface.
In conventional coupling, a light source, typically emitting a Lambertian distribution of light, is positioned adjacent to the edge of a planar waveguide element. The amount of light that directly strikes the coupling surface of the waveguide in this case is limited due to the wide angular distribution of the source and the relatively small solid angle represented by the adjacent planar surface. To increase the amount of light that directly strikes the coupling surface, a bare component such as the Cree ML-series or MK-series (manufactured and sold by Cree, Inc. of Durham, N.C., the assignee of the present application) may be used. A bare component is a light source that does not include a primary optic, lens, or discrete coupling optic formed about an LED chip. The flat emitting surface of the LED chip may be placed in close proximity to the coupling surface of the waveguide. While this arrangement helps ensure a large portion of the emitted light is directly incident on the waveguide, overall system efficiency generally suffers as bare components are typically less efficient than components having primary lenses, which facilitate light extraction from the component, improving overall efficiency.
As discussed above, the use of higher-efficiency LED elements having conventional (e.g., predominantly hemispherical or cubic) primary optics results in a limited amount of light that is directly incident on the coupling surface of the waveguide. Such light source(s) are often placed in a reflective channel or cavity to reflect light onto the coupling surface, thereby increasing the amount of light from the source that reaches the waveguide but also reducing overall system efficiency due to the loss incurred at each reflection event. In some luminaires, the waveguide(s) may have coupling surfaces specifically shaped to maximize the amount of light captured at the coupling surfaces. Alternatively, each LED may be positioned in a cylindrical coupling cavity within the waveguide, and a reflective cap having a cone-shaped plug diverter may be placed at the opposite end of the coupling cavity.
After light has been coupled into the waveguide, it must be guided and conditioned to the locations of extraction. The simplest example is a fiber-optic cable, which is designed to transport light from one end of the cable to another with minimal loss in between. To achieve this, fiber optic cables are only gradually curved and sharp bends in the waveguide are avoided. In accordance with well-known principles of total internal reflection, light traveling through a waveguide is reflected back into the waveguide from an outer surface thereof, provided that the incident light does not exceed a critical angle with respect to the surface. Specifically, the light rays continue to travel through the waveguide until such rays strike an index interface surface at a particular angle less than an angle measured with respect to a line normal to the surface point at which the light rays are incident (or, equivalently, until the light rays exceed an angle measured with respect to a line tangent to the surface point at which the light rays are incident) and the light rays escape.
In order for an extraction element to remove light from the waveguide, the light must first contact the feature comprising the element. By appropriately shaping the waveguide surfaces, one can control the flow of light across the extraction feature(s) and thus influence both the position from which light is emitted and the angular distribution of the emitted light. Specifically, the design of the coupling and distribution surfaces, in combination with the spacing (distribution), shape, and other characteristic(s) of the extraction features provides control over the appearance of the waveguide (luminance), its resulting angular distribution of emitted light (illuminance), and system optical efficiency.
In designing waveguide/extractor lighting systems, an important consideration is the purpose and/or positioning of the luminaire relative to the viewer and the illuminated surfaces. For example, in general illumination systems such as troffer lighting, the light source or luminaire is typically located on or near the ceiling and provides illumination to the walls and floor of a room. In this case, it is desirable for the luminaire to provide light in useful directions (e.g., towards the surfaces to be illuminated), and the ‘viewer’ or room occupant will typically be able to directly view the light emitting surfaces such that glare may become an issue if too much light is provided into a particular viewing angle from a sufficiently small emitting area. While low luminaire cost and architectural designs may call for smaller light-emitting surfaces, the requirement for limiting glare will typically place a lower limit on luminaire size and/or require architectural features such as recessing lighting to achieve the desired level of lighting. Alternately, conventional work or task lighting provides a light source that is necessarily offset from the observer's line of sight to prevent the light source from obscuring the object being viewed (e.g., a ring of lights around a microscope lens or a head light mounted above or to the side of a viewer's head). The light of such work lighting is angled toward the viewer's line of sight but is not in line with his/her sight. This offset lighting creates shadows and prevents a viewer from observing certain surfaces, such as the interior of narrow openings. Further, conventional work lighting also typically requires a large amount of light emitted from a necessarily small source, making the light source extremely visible and creating bright points or glare along the exiting surface of the work light as well as from reflections of the work light from shiny or reflective work surfaces.
Hulse U.S. Pat. No. 5,812,714 discloses a waveguide bend element configured to change a direction of travel of light from a first direction to a second direction. The waveguide bend element includes a collector element that collects light emitted from a light source and directs the light into an input face of the waveguide bend element. Light entering the bend element is reflected internally along an outer surface and exits the element at an output face. The outer surface comprises beveled angular surfaces or a curved surface oriented such that most of the light entering the bend element is internally reflected until the light reaches the output face
Parker et al. U.S. Pat. No. 5,613,751 discloses a light emitting panel assembly that comprises a transparent light emitting panel having a light input surface, a light transition area, and one or more light sources. Light sources are preferably embedded or bonded in the light transition area to eliminate any air gaps, thus reducing light loss and maximizing the emitted light. The light transition area may include reflective and/or refractive surfaces around and behind each light source to reflect and/or refract and focus the light more efficiently through the light transition area into the light input surface of the light-emitting panel. A pattern of light extracting deformities, or any change in the shape or geometry of the panel surface, and/or coating that causes a portion of the light to be emitted, may be provided on one or both sides of the panel members. A variable pattern of deformities may break up the light rays such that the internal angle of reflection of a portion of the light rays will be great enough to cause the light rays either to be emitted out of the panel or reflected back through the panel and emitted out of the other side.
Shipman, U.S. Pat. No. 3,532,871 discloses a combination running light reflector having two light sources, each of which, when illuminated, develops light that is directed onto a polished surface of a projection. The light is reflected onto a cone-shaped reflector. The light is transversely reflected into a main body and impinges on prisms that direct the light out of the main body.
Simon U.S. Pat. No. 5,897,201 discloses various embodiments of architectural lighting that is distributed from contained radially collimated light. A quasi-point source develops light that is collimated in a radially outward direction and exit means of distribution optics direct the collimated light out of the optics.
Kelly et al. U.S. Pat. No. 8,430,548 discloses light fixtures that use a variety of light sources, such as an incandescent bulb, a fluorescent tube and multiple LEDs. A volumetric diffuser controls the spatial luminance uniformity and angular spread of light from the light fixture. The volumetric diffuser includes one or more regions of volumetric light scattering particles. The volumetric diffuser may be used in conjunction with a waveguide to extract light.
Dau et al U.S. Pat. No. 8,506,112 discloses illumination devices having multiple light emitting elements, such as LEDs disposed in a row. A collimating optical element receives light developed by the LEDs and a light guide directs the collimated light from the optical element to an optical extractor, which extracts the light.
A.L.P. Lighting Components, Inc. of Niles, Ill., manufactures a waveguide having a wedge shape with a thick end, a narrow end, and two main faces therebetween. Pyramid-shaped extraction features are formed on both main faces. The wedge waveguide is used as an exit sign such that the thick end of the sign is positioned adjacent a ceiling and the narrow end extends downwardly. Light enters the waveguide at the thick end and is directed down and away from the waveguide by the pyramid-shaped extraction features.
Low-profile LED-based luminaires have recently been developed (e.g., General Electric's ET series panel troffers) that utilize a string of LED components directed into the edge of a waveguiding element (an ‘edge-lit’ approach). However, such luminaires typically suffer from low efficiency due to losses inherent in coupling light emitted from a predominantly Lambertian emitting source such as a LED component into the narrow edge of a waveguide plane.
Beeson et al. U.S. Pat. No. 5,396,350 teaches a backlighting apparatus used for flat panel electronic displays. The apparatus includes a slab waveguide that receives light from a light source positioned adjacent a side surface thereof and an array of microprisms attached to a face of the waveguide. Each microprism has a side surface tilted at an angle from the direction normal to the surface of the waveguide. Light emitted from the microprisms is substantially perpendicular to the slab waveguide.
Zimmerman et al. U.S. Pat. No. 5,598,281 discloses a backlight assembly for electro-optical displays. Light emitted from a light source disposed within a reflector travels through an array of apertures and is collimated by an array of tapered optical elements aligned with the array of apertures. Microlenses may be disposed adjacent the optical elements to further collimate the light. The surfaces of the optical elements are planar or parabolic in shape.
Zimmerman et al. U.S. Pat. No. 5,428,468 teaches an optical illumination system for applications that require substantially collimated light. The system comprises a waveguide that receives light from an edge thereof. An array of microprisms is attached to one face of the waveguide. Each microprism has at least two sidewalls tilted at an angle from the normal of the surface of the waveguide. An array of microlenses may be disposed atop the array of microprisms to further collimate the light.
Steiner et al. U.S. Pat. No. 5,949,933 discloses an optical illumination system for collimating light. The system includes a waveguide that receives light from an edge thereof and an array of lenticular microprisms attached to one face of the waveguide. Each microprism has a light input surface optically coupled to the waveguide and a light output surface opposite the input surface. The light input surface includes a number of tapered grooves perpendicular to the length of the lenticular microprism. The system also includes an array of microlenses to further collimate the light.
Hou et al. U.S. Pat. No. 5,839,823 teaches an illumination system including a light source adjacent to or housed within a reflector. A light-directing assembly having at least one microprism carried on a base wall is positioned adjacent the light source opposite the reflector. The microprism may be polyhedronal, curvilinear, and polyhedronal curvilinear. A lens array may be disposed on the other side of the base wall.
Kuper et al. U.S. Pat. No. 5,761,355 discloses a light directing optical structure comprising a waveguide having a multiplicity of prisms attached thereto. Light redirected by the prisms is constrained to a range of angles. The side face(s) of the prisms may be planar or curved. An array of lenses may be used to spread the light output of the prisms to a wider distribution angle